Syndiotactic polymer chain

The increase in size of the syndiotactic rrrr pentad peak of the boiling heptane soluble fractions was unexpected in view of the decrease in the rrrr pentad of unfractionated PP and the boiling heptane insoluble fraction (see Fig. 6). One possible explanation of this apparently unreasonable result is the following an external alkoxy silane donor preferentially deactivates active centers that produce mainly atactic polymer while leaving active those centers that produce mainly short chain syndiotactic polymer. Simultaneously there is an increase in the production of isotactic polymer with some stereochemical defects, but... 

Syndiotactic polymer (Section 7 15) Stereoregular polymer in which the configuration of successive chirality centers alternates along the chain... 

Homopolymerization of butadiene can proceed via 1,2- or 1,4-additions. The 1,4-addition produces the geometrically distinguishable trans or cis stmctures with internal double bonds on the polymer chains, 1,2-Addition, on the other hand, yields either atactic, isotactic, or syndiotactic polymer stmctures with pendent vinyl groups (Eig. 2). Commercial production of these polymers started in 1960 in the United States. Eirestone and Goodyear account for more than 60% of the current production capacity (see Elastomers, synthetic-polybutadiene). 

Substituents on the a-carbon atom restrict chain flexibility but, being relatively small, lead to a significantly higher Tg than with polyethylene. Differences in the Tg s of commercial polymers (approx. 104°C), syndiotactic polymers (approx. 115°C) and anionically prepared isotactic polymers (45°C) are generally ascribed to the differences in intermolecular dipole forces acting through the polar groups. 

Syndiotactic (Section 31.2) A chain-growth polymer in which the substituents regularly alternate on opposite sides of the backbone. 

In syndiotactic polymers the configurations of the steric centers on the backbone alternate. The net result is that side groups are positioned on the opposite side of the chain from their nearest neighbors, as illustrated schematically in Fig. 1.8 b). 

Recently, a similar analysis of the conformational energy has been performed also for various new syndiotactic polymers.27,47 The conformational energy maps of syndiotactic polypropylene (sPP),48 polystyrene (sPS),49 poly butene (sPB),25 and poly(4-methyl-l-pentene) (sP4MP)26 are reported in Figure 2.12. A line repetition group s(M/N)2 for the polymer chain, and, hence, a succession of the torsion angles. .. 0i, 0i, 02, 02,..., has been... 

Models of the chain conformations found in the various polymorphic forms of syndiotactic polymers are reported in Figure 2.13. 

When A A or B B, the symmetry is lower and the only possible line repetition groups are s(M/N) and tc for isotactic and syndiotactic polymers, respectively, in both cis and trans configurations. In these cases, two independent torsion angles in the main chain define the regular conformation (Oi and 02 in Figure 2.15). 

Mesomorphic forms characterized by conformationally ordered polymer chains packed in lattices with different kinds of lateral disorder have been described for various isotactic and syndiotactic polymers. For instance, for iPP,706 sPP,201 sPS,202 syndiotactic poly(p-methylstyrene) (sPPMS),203 and syndiotactic poly(m -methylstyrene),204 mesomorphic forms have been found. In all of these cases the X-ray fiber diffraction patterns show diffraction confined in well-defined layer lines, indicating order in the conformation of the chains, but broad reflections and diffuse haloes on the equator and on the other layer lines, indicating the presence of disorder in the arrangement of the chain axes as well as the absence of long-range lateral correlations between the chains. 

Polymers in which chain is made up of units having opposite space configuration of each asymmetric carbon atom are known as syndiotactic polymer. 

According to the scheme of Figure 12, in the framework of a regular chain-migratory mechanism the C2 and some Cs symmetric metallocenes lead to iso- and syndiotactic polymers, respectively. In fact, for the C2 symmetric systems the same propene enantioface is enchained at each... 

Figure 10.8. Chain end control through 2,1 insertions leading to syndiotactic polymer...

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